They cause me all sorts of grief in troubleshooting.
That said, I think they are one of those things that if you really take a lot of time to plan things out, they are useful. A LOT of us simply use NS almost in a reactionary state - get whatever done you have to, ASAP. But there are a lot of other companies where their processes are all VERY structured. For instance, I had one company with a 4 hr window each month on 1 day to get all patching done including reboot. That's it. For companies like this, where everything is sooooo structured, maintenance windows can be useful.
I'm personally finding a lot of people taking them off, but there it is. :-)
Oh, and many tasks can be set to bypass maintenance windows, to help prevent what you complained about. Not all..